Cancer results from the subversion of processes that control the normal growth, location and mortality of cells. This loss of normal control mechanisms arises from the acquisition of mutations that lead to the oncogenic activation of proteins that are involved in the normal regulation of such processes.
Protein kinases are enzymes that catalyse the transfer of phosphate from adenosine-5′-triphosphate (ATP) to specific amino acid residues in many proteins. Generally, the phosphorylation of a protein changes its functionality, from inactive to active in some cases, and from active to inactive in others. Protein kinases are thus involved in the regulation of many aspects of cell function, as most of the signal transduction pathways controlling cell growth, survival, differentiation and motility are mediated by phosphorylation. Abnormal activity of protein kinases has been implicated in many cancers as well as in other diseases. The human genome encodes at least 518 kinases, of which approximately 90 specifically phosphorylate the phenolic hydroxyl of tyrosine residues. Tyrosine kinases are particularly involved in cell proliferation and survival processes, and their aberrant activation most often leads to oncogenic transformation.
For example, structural alterations in ALK produced by the chromosomal rearrangement t(2q23;5q35) generates the NPM/ALK oncogenic fusion protein associated with ALCL (Rabbitss, T. H. Nature, 1994,372, 143).
Large cell lymphomas represent about 25% of all non-Hodgkin's lymphomas; about one-third of these tumors are anaplastic large cell lymphoma (ALCL). In turn, the majority of ALCL patients (60-80%) possess a chromosomal translocation that leads to the in-frame juxtaposition of the 5′ portion of the nucleophosmin (NPM) gene with the sequence encoding for the catalytic domain of ALK kinase. The resulting chimaeric gene, under the control of the strong NPM promoter, drives the expression of the NPM/ALK oncogenic fusion protein. An additional 10% of ALCL patients carry other ALK fusion proteins. To date, 11 ALK fusions have been described. In all cases, the ALK kinase domain sequence is fused to an aminoterminal protein-protein interaction domain of a protein that is highly expressed in the target cell. Thus, the fusion partner provides constitutive expression (through its promoter) and activation (via oligomerisation). In addition, ALK fusion proteins show anomalous cellular localisation. For example, NPM/ALK is mainly found in the cytoplasm and the nucleus. By contrast, wild-type ALK is a tightly regulated, integral membrane protein that is only activated in the presence of a specific extracellular ligand.
About 5-8% of NSCLC patients carry the EML4/ALK fusion. As with NPM/ALK, the 5′ fusion partner EML4 provides high expression and constitutive activation of the ALK kinase. The population of ALK+NSCLC patients, although representing a minority of all NSCLC patients, is estimated to be about 50-70,000 new cases worldwide each year. In addition to fusion proteins, activating point mutants of ALK have been described and validated in familial (90% of cases) and sporadic (˜10%) neuroblastoma and in anaplastic thyroid carcinoma (10% of patients).
ALK is normally expressed in the nervous system during embryonic development and is strongly down-regulated at birth, resulting in barely detectable levels in adult tissues. It has been extensively demonstrated that constitutively active NPM/ALK is a potent oncogene with transforming and tumourigenic properties (Morris et al., Science, 1994, 263,1281-1284).
Moreover, rearrangement of ALK kinase is a very early event in tumour formation and is necessary for survival of transformed cells. The high level of expression of NPM/ALK and other ALK fusion protein variants in lymphoma cells and their direct role in lymphomagenesis, combined with the fact that normal ALK is expressed at low levels in the human body, suggests that ALK could potentially be an ideal target for therapy.
There is currently only one drug clinically available for the treatment of ALK-positive cancer. Crizotinib is a dual MET/ALK inhibitor recently approved for ALK+ NSCLC. It potently inhibits ALK phosphorylation and induces apoptosis in ALK+ cancer cells. Initial clinical trials showed excellent activity and tolerability in advanced NSCLC patients (Shaw et al., Lancet Oncol 2011; 12: 1004-12) However, clinical resistance develops in a significant fraction of patients (Choi et al., N Engl J Med 2010; 363: 1734-9). At least half of the patients show either amplification of ALK gene or acquisition of a secondary mutation that renders ALK insensitive to Crizotinib. In particular, the gatekeeper mutant L1196M showed high resistance to Crizotinib. Therefore, there is urgent need for second-generation compounds, with higher potency and selectivity, able to inhibit Crizotinib-resistant mutants and to circumvent clinical resistance. Moreover, it would be desirable to develop compounds which are non-ATP competitive.